The 27-year-old left-back moves on loan initially with a three-year contract set to follow in the summer.
As the closure of the transfer window draws closer, former Coventry City, Celtic and Burnley man Fox, joins Algerian forward Rafik Djebbour as a confirmed arrival at the City Ground.
Update: French central defender Kevin Gomis is signing number three, on loan from OGC Nice until the end of the season subject to international clearance.